Card with metal layer and an antenna

ABSTRACT

In a smart card having an antenna structure and a metal layer, an insulator layer is formed between the antenna structure and the metal layer to compensate for the attenuation due to the metal layer. The thickness of the insulator layer affects the capacitive coupling between the antenna structure and the metal layer and is selected to have a value which optimizes the transmission/reception of signals between the card and a card reader.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the continuation application of U.S. applicationSer. No. 14/543,495 filed Nov. 17, 2014, now U.S. Pat. No. 9,721,200issued Aug. 1, 2017, which claims priority to U.S. ProvisionalApplication No. 61/962,873 filed Nov. 18, 2013, the contents of eachapplication incorporated by reference herein.

BACKGROUND OF THE INVENTION

This invention relates to smart cards and in particular to multi layeredsmart cards having a metal foil/film/layer and an antenna structure forreceiving or transmitting signals between the smart cards and a cardreader.

As shown in FIGS. 1, 2(a), 2(b) and 2C a typical smart card includes anantenna structure coupled (directly or electromagnetically) to amicroprocessor or microcomputer (also referred to as a “chip”) alsolocated on or within the card. The antenna structure functions to enablecontactless communication between a card reader (also referred to as a“transponder” or “transceiver”) and the microprocessor. That is: (a)signals emitted by the card reader are electromagnetically coupled viathe antenna to the chip which receives and processes the signals; and(b) signals processed and emitted by the chip are electromagneticallytransmitted via the antenna to the card reader. In the manufacture ofcertain smart cards it is highly desirable that a metal foil/film/layerbe included among the card layers. However, the metal foil/film/layerpresents a problem since it functions to attenuate (absorb) the signalstransmitted between the card reader and the chip limiting communicationor even making it impossible. As an example, it is a requirement that acard reader should be able to read a smart card located at a givendistance from the card reader. Known cards with a metal foil/film/layercould not be read reliably at these established minimum requireddistances.

To overcome the attenuating effect of the metal foil/film/layer, aninsulator layer may be formed between the metal film/foil/layer and theantenna structure. A conventional approach is to make the insulatorlayer very thick to decrease the attenuating effect of the metalfoil/layer. However, this is not acceptable where the permissiblethickness of the insulator layer is limited. As is known in the artthere are numerous requirements which have to be met in the manufactureof cards. Some go to the structural integrity of the cards (e.g., theyshould not bend, delaminate) and be capable of use for several years anda large number of user cycles. So, the cards need to be formed usingnumerous layers with various requirements on the thickness andcomposition of the layers. Thus, it is not satisfactory to just make theinsulator layer arbitrarily very thick since such thick layers interferewith other requirements in the manufacture of smart cards.

The problem of manufacturing a reliable smart card is even greaterparticularly when the card includes a metal layer which interferes withthe transmission/reception of signals between the smart card and a cardreader.

Thus, a significant problem faced by Applicants related to selecting thethickness of an insulator layer, extending within a prescribed range,which can provide reliable readings of card data by a card readerlocated at a prescribed distance and at a prescribed frequency ofoperation. An associated problem was finding a thickness for theinsulator layer which provided improved transmission/reception.

SUMMARY OF THE INVENTION

Applicants invention resides, in part, in the recognition that thecombination of an antenna structure and its associated electronics,including an RFID chip, formed on or within a card can be tuned byvarying the distance (“d”) between the conductive wires forming theantenna structure and a metal film/foil/layer. “Tuning” as used hereinincludes enhancing the transmission/reception of signals between thecard's antenna structure and a card reader (or like device), so thatreliable communication can be had between the card and the card readerat predetermined distances and at predetermined frequencies. Inaccordance with the invention, “tuning” can be achieved by controllingthe thickness of an insulating (non-conductive) layer or layers formedbetween the antenna structure and the metal film/foil/layer.

In fact, Applicants discovered that, in response to certain transmittedcard reader signals, the amplitude of the signals received at the card'santenna were greater for some insulator thickness, which may be termedthe preferred thickness (“Tp”), than for thicker insulator layers.Varying the thickness of the insulator layer, varies the distance “d”between the antenna structure and the metal layer and controls thecapacitance between the antenna structure layer (23) and the metal layer(24). Varying the capacitance can be used to “tune” the structure toimprove the read/write distance between a card reader and a card and theperformance (reception/transmission) of the inductive coupling system.

The antenna or antennas can be formed on either side of the holographicmetal foil/film/layer and the resulting finished card body can, for thinmetal film/foil layers, be interrogated by a card reader from eitherside of the card body.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, which are not drawn to scale, likereference characters denotes like components; and

FIG. 1 is a cross sectional diagram illustrating the placement of anantenna structure in close proximity to a metal film located about thecenter of a card;

FIG. 2A is a top view of a card layer including an antenna structure anda chip in accordance with one embodiment of the invention;

FIG. 2B is a top view of a layer including a metal layer or holographicfilm structure in accordance with one embodiment of the invention;

FIG. 2C is an exploded view of an insulator layer formed between anantenna and chip bearing layer (above) and a metal layer (below);

FIG. 2D is a highly simplified equivalent circuit illustrating thefunction and effect of a metal foil on the transmission/reception ofsignals between a transponder (RF transmitter/receiver) and an RFIDchip;

FIG. 3 shows the stacking of some of the layers to form card;

FIG. 4 illustrates the use of an RF transmitter to transmit RF signalsto a card structure in accordance with the invention; and

FIG. 5 is a bar graph showing results of measurements of signalamplitude sensed at the antenna of the structure shown in FIGS. 1 and 4.

DETAILED DESCRIPTION

Referring to FIGS. 1 through 4 there is shown a generally symmetricaltype of card structure illustrating the use of buffer layers in a cardconstruction to absorb the difference between layers havingsubstantially different characteristics which enables many differenttypes of sturdy and secure cards to be manufactured. Card 5 includes (inthe Figures) a top section 101 a, a correspondingly symmetrically shapedbottom section 101 b and a center section 103.

Top section 101 a includes a PVC overlay 14 a mounted over a core PVClayer 10 a, overlying a buffer layer 22 a, which, in turn, overlies aPET layer 12 a, overlying a buffer layer 25 a.

Bottom section 101 b includes a PVC overlay 14 b formed under a core PVClayer 10 b, underlying a buffer layer 22 b, which underlies a PET layer12 b, underlying a buffer layer 25 b. The buffer layers reduce thestress between the very dissimilar materials enabling a more stablestructure having a much greater life time and of greater sturdiness.

The center section 103 of card 5 formed between sections 101 a and 101 bincludes a metal layer 24 separated by an insulator layer 21 from theantenna structure layer 23.

Metal layer 24 may be a holographic film or a metal foil. The metallayer may be used in the card for decorative purposes to give the card ametallic or rainbow coloring which reflects light in desirable ways. Or,to serve any other functional or cosmetic purpose, The metal film/foillayer 24 may be may be metalized or transparent, holographic or plainmetalized non-holographic material. The thickness of the metal layer mayrange from about 1 micron to 200 microns.

The antenna structure layer 23 may include an antenna directly coupledto an RFID chip. Alternatively, it may include a booster antennainductively coupled to a chip antenna which in turn is directly coupledto an RFID chip 206. The antenna structure 23 may be part of an “RFIDinlay” which includes an RFID chip and associated antenna(s) tocommunicate with a reader or like device. The antenna structure may beformed on or within a suitable plastic layer. The antenna structure 23and its associated RFID chip 206 (which may be on same layer or ondifferent layers) are intended to communicate wirelessly with a cardreader 100. The metal layer 24 interferes with the wirelesscommunication as it attenuates (absorbs) the RF energy transmittedbetween the card reader and the card.

In cards embodying the invention, the insulator layer 21 is insertedbetween the antenna structure and the metal layer to counteract theattenuation due to the metal layer. The insulator layer may be formed ofany material capable of insulating near field high frequency (HF)signals, but is typically PVC, PET, PETG, PC, latex, cellulose,fiberglass (Teslin), an adhesive and/or composites of these, or otherpolymers used in card construction. The thickness of the insulator layer21 may typically range from 10 microns to 350 microns. The maximumthickness of the insulator layer is normally set by constrainspertaining to the various layers used to form the card. Within thepermissible, or allowable, range of thicknesses there may be an optimumvalue of thickness which provides best results, as discussed below, andwhich, in accordance with invention, may be the selected value ofinsulator thickness. As discussed below, the thickness of the insulatormay be selected to have a value which will optimize (tune) thetransmission and reception of signals between a card reader 100 and theantenna structure or chip 206.

In one embodiment the various layers of the rd of FIG. 4 wereapproximately as follows:

(i) PVC laminate layers 14 a, 14 b were 2 mils each,

(ii) The core PVC layers 10 a and 10 b were 6 mils each,

(iii) The PET layers 12 a and 12 b were 1 mils each, and

(iv) The buffer layers 22 a, 22 b, 25 a, and 25 b were 1 mil each

(v) The antenna structure 23 and RFID chip layer were 2 mils

(vi) The metal layer 24 was 2 mils; and

(vii) The insulator layer was 3 mils. Note that depending uponproperties desired and cost constraints polyester layers (the moreexpensive material) can be placed near both outside surfaces or in thecenter or unbalanced in the core such that a “bowed” card can bestraightened after personalization with a clear lamination. In general,PVC is used to form the outer layer of a card because it enables thepersonalization of a card to be made more easily. PVC is also normallyused because it enables thermal printing or embossing. PVC basedmaterials are normally much cheaper than PET, whereby the greater use ofPVC is desirable for economic reasons. Thus, the layers of PVC materialare normally thicker (individually and in the aggregate) than the layersof PET and of buffer material

Referring to FIGS. 1 through 4 note that a composite card 5 may beformed using numerous layers of different materials which serve variouspurposes as described below. Pertinent to the present invention is theformation of an inductive coupling antenna structure 23 in closeproximity to a holographic metal film/foil layer 24. In FIGS. 1 and 4,the metal film 24 is shown located in or near the center of the cardbody. However this is by way of illustration only and the antennastructure and metal film could be located near the top or bottom of thecard. The antenna structure 23 may be formed above a plastic layer orwithin a plastic layer. As shown in FIGS. 2A and 2C an RFID chip 206 maybe directly connected to the antenna. Alternatively, as shown in FIG. 4,the RFD chip 206 may be located at a different level (layer) than theantenna structure and be inductively (or directly) coupled to theantenna structure. Thus, smart cards embodying the invention include anantenna either directly connected or inductively connected to an RFIDchip to provide contactless communication with a card reader 100. Theterm antenna structure 23 as used herein may refer to the antenna and/orto the antenna and its associated circuitry including the chip.

As shown in FIG. 4, a card reader or an RF transmitter 100, can sendsignals to a smart card having an antenna structure 23 separated from ametal layer 24 by an insulator layer 21. The antenna structure 23 isinductively coupled to an RFID chip 206. As is known, a card reader (ortransponder) 100 is used to interrogate the card by sending(transmitting) RF signals to the card which are electromagneticallycoupled via the antenna structure to the RFID chip 206. In response tothe received interrogation signals, the card produces RF signals whichare in turn transmitted to the card reader where they are received forprocessing. As noted above, a problem with having a metal layer 24 isthat the metal layer attenuates (absorbs) the electromagnetic energyimpeding the transmission/reception of signals between the card and thecard reader.

The insulator layer 21 (which may be an adhesive layer or any suitableinsulating layer as noted above) is interposed between the antennastructure 23 and metal layer 24. An important aspect of the invention isthe selection of the thickness “d” of layer 21.

Applicants recognized that the antenna structure 23 and its associatedelectronics (e.g. RFID chip 206) can be tuned by varying the distance(“d”) between the conductive wires forming the antenna structure and theholographic metal film which was done by controlling the thickness oflayer 21. Varying the distance “d” controls the capacitance between theconductive elements in layers 23 and 24. This tuning can be used toimprove the read/write distance and performance of the inductivecoupling system. Varying the distance “d” can thus be described asseeking and finding a resonant or quasi-resonant frequency range.

Some insight into the interaction between the metal layer and theinsulator layer and their effect on signal transmission/reception may beobtained by reference to FIG. 2D.

FIG. 2D depicts the modelling of the role of the foil and the insulatorlayer and represents an attempt to illustrate and explain theinteractive effect of a metal foil and the role of an insulator layer,formed on a card, on the signal transmission between a card 204(equivalent to card 5 in FIG. 4) and a card reader 202 (equivalent tocard reader 100 in FIG. 4). The metal foil and the insulator layer maybe represented by a network 200 which includes capacitors C4 and C5 andresistors R4 and R5. The card reader may be represented by a transponder212 and associated components R1, C1 and R2 and an antenna coil L1included in network 202. The card antenna structure and associatedelectronics may be represented by antenna coil L2 resistor R3 capacitorsC2 and C3 and an RFID chip 206 all included in a network 204.

Resistors R4 and R5 change with foil (metal layer) thickness. Thethicker the metal the lower the value of resistance. The values of C4and C5 change as a function of the thickness “d” of the insulator layerbetween the metal layer and the antenna wires. The smaller (thinner) theinsulator thickness the higher is the capacitance of C4 and C5. Notethat the one end of resistors R4 and R5 connected in common are shownretuned to ground (GND). This ground (GND) is a “virtual” ground in sofar as it represents the grounding effect of the metal foil. That is,the metal foil absorbs the electromagnetic energy but it does not havean electrical path back to the chip or the transponder.

Although FIG. 2D provides insight into the interaction of the metal foiland the insulator layer, it does not enable a user to determine a rangeof values of C4 and C5 which can optimize transmission/reception at aspecified frequency and given metal thickness. It also does not enable auser to derive some form of an approximate equation yielding theresonant frequency.

In accordance with one aspect of the invention, a preferred set ofvalues for the thickness of an insulator layer was determined for aselected structure of the type shown in the figures, as follows. A setof four cards was fabricated which varied only in that the thickness (T)of insulator layer 21 was varied in steps. The antenna layer 23 of the 4sets of cards was the same and the metal layer of 50 microns was thesame for the 4 sets. The insulator layer thickness of the 4 sets wasvaried to be 50, 100, 150 and 200 microns, respectively. Signals weretransmitted from a transponder, such as RF transmitter 100, and theamplitude of the signals received at the antenna structure was thenmeasured for each card. As shown in FIG. 5, for signals emitted by thetransponder 100 at a nominal carrier frequency of 13.56 MHz, themeasured amplitude of the signals received at the antenna structure had:(a) a relative value of approximately 11 for an insulator thickness of50 microns; (b) a relative value of approximately 38 for a thickness of100 microns; (c) a relative value of approximately 80 for an insulatorthickness of 150 microns; and (d) a relative value of approximately 62for a thickness of 200 microns. This demonstrates that, for theparticular structure shown in the figures, an insulator thickness in therange of 150 microns provides better reception than a thicker (i.e., 200micron) insulator layer. This is a significant result demonstrating thattesting can reveal a range of values for the thickness of the insulatorlayer which provide better results than would be expected based on theassumption that the thickest insulator provides the best results fortransmission/reception of signals. Thus, as noted above, aquasi-resonant frequency range is located where improvedtransmission/reception is achieved.

For other card structures, a thickness “d” equal to 54 microns was foundto be acceptable.

In accordance with the invention, card structures containing a setantenna structure and a set metal layer (of predetermined thickness) canbe tested to ascertain the thickness of a separating insulator whichwill provide better or best reception between a card reader and theantenna structure.

(a) This can be done on an individual card basis.

(b) This method enables the manufacture of a few cards to determine bestoperating ranges and then follow through with the manufacture of batchesof cards.

(c) The method also enable a user to vary various parameters of the card(e.g., foil thickness and antenna structure) for a given insulatorthicknesses and determine reliable operating ranges.

(d) So, as used herein the “selected” or “preferred” insulator thickness(Tp) is defined as the insulator thickness Tp that gives the best (or atleast acceptable) reception (at the antenna) to a transmitted signalfrom a card reader. This value of Tp will also apply for the signalsemitted by the card chip via the antenna to the card reader.

(e) Tp may be determined (found) for the frequency (e.g., 13.56 MHz) atwhich the card reader (or interrogating device) is operating to ensurebest responses.

(f) Tp may be determined empirically for a given metal configuration andits thickness and a given antenna structure configuration. At this timeno satisfactory equation has been generated which can be used toaccurately predict Tp for different configurations.

(g) The value of Tp may be different if the antenna structure changesbecause the frequency response profile of the antenna will also change.The design of the antenna may have to change due to embossingrequirements or other physical attributes of the cards.

(h) The invention may be applicable to all types of metal cards. It isuseful with “bulk” metal cards as well when it includes a ferrite layer.So it is relevant to all card constructions.

(i) The invention is useful in making products in that for a given cardproduct there are physical constraints which would dictate the antennaform factor and some of its surrounding layers. Based on thoseconstrains, the insulating layer may be “tuned” as taught herein tomaximize the signal response between the card and a card reader.

(j) The invention is particularly useful since it teaches that given theconstraints applicable to a card product, the insulator layer an bevaried to optimize transmission/reception results. This goes beyond theprior art suggestion of just tuning the antenna.

The invention claimed is:
 1. A card comprising: a singular metal layer;an antenna structure; an insulator layer formed between the metal layerand the antenna structure, said insulator layer having first and secondsurfaces and a thickness, said first surface in direct contact with saidantenna structure and said second surface in direct contact with saidmetal layer, said insulator layer exhibiting a coupling capacitancebetween said antenna structure and said metal layer, said couplingcapacitance having a value that is a function of the thickness of theinsulator layer, said metal layer tending to attenuate the amplitude ofa radio frequency (RF) signal of a predetermined frequency received atthe antenna structure; wherein the insulator layer thickness is selectedto provide a maximum amplitude of the RF signal within a limited rangeof permissible insulator thicknesses.
 2. The card of claim 1, furthercomprising an RFID chip coupled to the antenna structure.
 3. The card ofclaim 1, wherein said card is configured to transmit and receive signalsto and from a card reader located at a predetermined distance from thecard.
 4. The card as claimed in claim 1, wherein said card includesadditional plastic and buffer layers.
 5. The card of claim 4, whereinthe card comprises a number of plastic layers above the antennastructure and a like number of plastic layers below the metal layer. 6.The card of claim 1, wherein the metal layer comprises a holographicfilm.
 7. The card of claim 1, wherein the metal layer comprises a metalfoil.
 8. The card of claim 1, wherein the selected insulator thicknessis less than a maximum permissible insulator thickness.
 9. The card ofclaim 8, wherein the range of permissible insulator thicknessescomprises a range from 10 microns to 350 microns.
 10. A card comprising:an antenna structure configured to receive a predetermined radiofrequency (RF) signal; an RFID chip coupled to the antenna structure; asingular metal layer that has a tendency to attenuate amplitude of theRF signal received at the antenna structure; and an insulator layer indirect contact between the antenna structure and the metal layer, saidinsulator layer functional to provide capacitance coupling between theantenna structure and the metal layer and having a thickness thataffects the value of capacitance between said antenna structure and saidmetal layer, said insulator layer thickness selected to provide aresonant or quasi-resonant response to the predetermined RF frequency.11. The card of claim 10, wherein the selected insulator layer thicknessis an optimal value that maximizes the RF signal amplitude of the signalreceived at the antenna structure within a range of permissibleinsulator thicknesses.
 12. The card of claim 11, wherein the optimalvalue is a function of the antenna structure, thickness of the metallayer, and the predetermined RF frequency.
 13. The card of claim 11,wherein the selected insulator thickness is less than a maximumpermissible insulator thickness in the range of permissible insulatorthicknesses.
 14. The card of claim 10, wherein the selected insulatorthickness comprises a thickness in a range of 10 microns to 350 microns.15. The card as claimed in claim 10, wherein said card includesadditional plastic and buffer layers.
 16. The card of claim 15, whereinthe card comprises a number of plastic layers above the antennastructure and a like number of plastic layers below the metal layer. 17.The card of claim 10, wherein the predetermined RF signal has afrequency associated with a card reader for transmitting and receivingsignals to and from the card at a predetermined distance from the card.18. The card of claim 17, wherein the predetermined RF signal has afrequency of 13.56 MHz.
 19. The card of claim 10, wherein the insulatorlayer comprises a material functional to insulate near-field, highfrequency (HF) signals.
 20. The card of claim 19, wherein the materialis selected from the group consisting of: PVC, PET, PETG, PC, latex,cellulose, fiberglass, an adhesive, and composites thereof.